This invention relates to a method of detecting a possible leak in a vacuum package, which comprises
placing the package in a gauging space, PA1 shutting off the gauging space, PA1 using a pressure in the sealed gauging space that is higher than that within the vacuum package, and PA1 gauging the pressure in the gauging space as a function of the time for a predetermined gauging period. PA1 at least two equally sized gauging spaces, each for receiving an identical vacuum package, said spaces being connectable to a common space, PA1 means for sealing each gauging space off the common space, PA1 means for simultaneously gauging the pressure as a function of the time in each gauging space, and PA1 means for determining the difference in pressure variation between the two or each pair of gauging spaces.
A method of this kind is known from European Pat. No. 152,981, which describes a method wherein the vacuum package to be tested is placed in a gastight, sheet-like body. After this body has been hermetically sealed, the pressure in the space between the sheet-like body and the package is gauged for some time as a function of the time. The change in pressure can be used as a measure for establishing whether there is a leak in the package.
In certain cases, it had been found that it was not possible with this method to identify leaky packages in a short period with sufficient reliability. In part, this inaccuracy can be attributed to the usually minute size of an occurring leak. When, for instance, a package of coffee with a vacuum pressure of about 40 mbar is leaky, it may sometimes take many days in atmospheric surroundings before the pressure in the package has increased to a value of about 800 mbar, at which pressure the package has softened palpably.
For economic and practical reasons, it is desirable to keep the gauging time short, e.g. not longer than 10 seconds. In this short period of time, the rise in pressure in the leaky package of the example mentioned will, e.g., be as little as about 0.09 mbar. The corresponding pressure drop in a gauging space of about equal magnitude as the free space in the package and at atmospheric initial pressure will then be approximately of the same order of magnitude, e.g. 0.1 mbar. Pressure gauges adapted to detect such a minute change in pressure are commercially available. At a gauging time shorter than 10 seconds, the change in pressure will naturally be smaller still.
Apart from this weak leakage signal in the gauging space, it turns out that there are other causes for changes in pressure in this space, in particular those resulting from temperature changes during the gauging of a package having a temperature differing from the ambient temperature. The influence of these disturbances relatively to the leakage signal may be comparatively substantial. As a result, the changes in pressure per unit of time in the gauging space as a result of even slight temperature changes of the package, e.g. a temperature drop of one-tenth or some tenths of degrees Celsius during the measuring period, may already lead to pressure disturbances that are up to many times higher than a possible leak signal. The disturbances can result both in a pressure drop and a pressure rise in the gauging space, with the eventual magnitude being the resultant of a plurality of separate disturbing influences. Also, the magnitude of the pressure disturbances is not always constant during a longer period on which many hundreds of packages are successively tested for leaks.
In addition to temperature influences, other disturbing factors may occur, e.g. a disturbance factor that may occur in a gauging method according to the above mentioned European Pat. No. 152,981, wherein the gauging space is formed on the exterior by a flexible bag pressed at superatmospheric pressure against the vacuum package placed in the bag. It has been found that the abutment of this bag against the package can slightly change during gauging as a function of the time, thereby also changing the size of the gauging space remaining between package and bag in a disturbing manner.